Field Of The Invention
The present invention generally concerns variable-area flowmeters and mechanical flow regulators. The invention particularly concerns highly accurate flowmeters and flow regulators that are substantially insensitive to changes in the viscosity of fluids.
The invention further concerns the use of viscosity-insensitive flowmeters and flow regulators in systems for mixing two fluids together in prescribed relative proportions and to a predetermined total volume, particularly for mixing together and dispensing carbonated water and flavored soft drink syrup in a soft drink dispenser.
Prior Fluid Dispensing Systems
Soft drinks are typically made by mixing a syrup with carbonated water. The dispensers typically inject the water and syrup simultaneously into a mixing chamber where they are mixed together. The mixed fluids are then dispensed through a nozzle into a drinking cup. The two fluids are normally supplied for coextensive time durations, and the mix ratio is typically controlled using manually-adjustable metering pins.
To keep the quality of the mixed drinks at a desired level, the syrup and water have to be mixed with a very high degree of accuracy. Syrup is desirably mixed with water in a predetermined proportion, typically from 1/7 to 1/3 syrup to water. Syrup manufacturers, and sellers of soft drinks, believe that the accuracy of the mixing, or "brix", should desirably be not worse than .+-.1%. Variations from the desired mix accuracy result in uneconomical use of the syrup component, and undesirable variations in the quality of the mixed drink product.
The desired mixing precision has not yet been achieved in many soft drink dispensers. One source of error in mixing precision is due to variations in the absolute, and relative, pressures of the syrup and of the water during the period of dispensing operation, and between dispensing periods. These pressure variations tend to alter the absolute flow rate of the syrup and/or the water, and the flow rate of the syrup relative to the flow rate of the water. These varying flow rates, in turn, change the mixing, or "brix" of the beverage.
To maintain a fixed mix ratio, many soft drink dispensers provide for manual adjustment of flow control metering pins in the syrup and water supply lines. The adjustment procedure is time consuming and cumbersome. A special container called a "standard measuring cup" is employed. The standard measuring cup has two reservoirs calibrated by a single scale, typically two vertical cylinders with a centrally-located scale. The volumes of the two reservoirs per scale unit of fill are maintained in a fixed proportion, for example five to one (5:1). The syrup and water output flows are separated. Each flow is plumbed into a corresponding reservoir of the standard measuring cup. The flows are adjusted to be in proper proportion to each other by observing the filling of the standard measuring cup. Normally this is done by adjustment of the flow control metering pins. The adjustment procedure must be periodically repeated in order to account for any long term changes in the pressures or viscosities of the dispensed fluid. Short term variations in flow rates during a single dispensing operation, or between individual dispensing operations, cannot be corrected for satisfactorily by periodic manual adjustments.
Some soft drink dispensers provide for automatic adjustment of fluid flow rates. These dispensers generally are unduly complex. Most still require substantial manual adjustment when changing from one type of syrup to another.
Some dispensers employ a flow washer as a flow-regulating device. Flow washers are reasonably effective in precisely controlling flow rates over that range of pressures which occur during beverage dispensing operations. However, flow washers do not appropriately compensate for another factor which contributes to variations in the accuracy of the mixing operation, or "brix". This factor is the change in fluid friction resulting from changes in viscosity of the mixed fluids. The syrup, in particular, has a viscosity that exhibits a very high dependance on temperature, and that undergoes significant changes during the normal environmental temperature variations that occur during soft drink dispensing.
When the fluid friction, or viscosity, of either fluid component (e.g., water or syrup) changes, an equal amount of applied pressure causes a different amount of that fluid component to flow, thereby changing the relative proportions of the dispensed fluids.
Some soft drink dispensing systems attempt to measure fluid flow rates with flowmeters in order to adjust the flow rates, and to maintain them in a prescribed ratio. Unfortunately, the same fluid viscosity variations that cause problems with controlling fluid flow with flow washers also cause problems with flowmeters.
Syrups commonly vary in viscosity from .times.2 to .times.20 over the range between 32.degree. F. (0.degree. C.) and 194.degree. F. (90.degree. C.). These large variations cause commensurate variations in the signal outputs of viscosity-sensitive flowmeters. Dispensing systems that are dependent upon such viscosity-sensitive flowmeters for regulating fluid flow to dispense (i) a prescribed mix ratio, and/or (ii) a predetermined volume, are correspondingly subject to undesirable error in measurement.
One previous system that attempts to provide accurate proportions of dispensed fluids despite variations in fluid pressure and/or fluid viscosity is shown in U.S. Pat. No. 4,487,333 for a FLUID DISPENSING SYSTEM. The system employs syrup and water flowmeters in order to monitor the instantaneous flow rates of both the water and fluid. Responsive to this monitoring, separate syrup and water valves are controllably turned on and off, each independently at an appropriate duty cycle, in order to provide a prescribed mix ratio. Fluid flow is sensed and controlled directed to minimize the effects of pressure variations in the syrup and water supplies, and permit dispensing of the water and syrup in accurate and constant proportion.
However, the flowmeters of this previous fluid dispensing system are sensitive to changes in the viscosity of the fluid for which flow is sensed. Accordingly, the system attempts to accommodate for variations in the relative proportion of water and syrup in the dispensed mix that result from changes in fluid viscosity. In order to do so, a temperature sensor is used to sense the temperature of the syrup. A microprocessor-based digital electronics control circuit receives this sensed temperature, as well as the sensed flow rate. The control circuit references a separate, removable, personality module for each particular type of syrup. The module contains information on both the prescribed mix ratio for that particular syrup and the dependence of the syrup's viscosity on changes in temperature. The control circuit uses this information to calculate the appropriate duty cycles that will control for pressure variations, and also for viscosity variations, in the dispensed syrup component.
This type of fluid dispensing system is generally complex and expensive. It performs its dispensing tasks using predetermined information that is stored in the removable personality module. The information typically includes mix ratios and viscosity characteristics of the syrup component. The contents of the personality module is customized for each type of syrup--often, a difficult task.
The fluid dispensing system of U.S. Pat. No. 4,487,333 operates to control fluid control fluid flow by cycling a solenoid valve on and off at a variable duty cycle. The cycling introduces unwanted noise and vibration into the system.